Ch 16: Acid-Base Equilibria

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Ch 16 Acid-Base Equilibria

• Brown, LeMay Ch 16
• AP Chemistry

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16.1 Acids and Bases

• Defined by Svante Arrhenius in 1880s
• Arrhenius acids produce protons increase H
• HCl (aq) ? H (aq) Cl- (aq)
• Arrhenius bases produce hydroxides increase
  OH-
• NaOH (aq) ? Na (aq) OH- (aq)
• or
• NH3 (aq) H2O (l) ? NH4 (aq) OH- (aq)

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16.2 Dissociation of Water

• Autoionization of water
• H2O (l) ? H (aq) OH- (aq)

• KW ion-product constant for water
• H3O (aq) or H (aq) hydronium

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16.3 The pH Scale
pX -log X

• pH -log H -log H3O
• or H 10-pH

pOH -log OH- or OH- 10-pOH
HOH- KW 1.0x10-14 -log (HOH-)
-log KW -log H -logOH- -log
(1.0x10-14) pH pOH 14.00
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16.3 The pH Scale
pH 14 7 0
pOH 0 7 14

• If HltOH-, then Hlt1.0x10-7
• Ex pH -log1.0x10-10 10.00 (basic)
• If H OH-
• Since HOH- 1.0x10-14
• H OH- 1.0x10-7
• pH -log1.0x10-7 7.00 (neutral)
• If HgtOH-, then Hgt1.0x10-7
• Ex pH -log 1.0x10-3 3.00 (acidic)

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16.4 Brønsted-Lowry Acids Bases

• Johannes Brønsted (Denmark)
• Thomas Lowry (England), 1923
• Brønsted-Lowry acids H donor
• Brønsted-Lowry bases H acceptor
• NH3 (aq) H2O (l) ? NH4 (aq) OH- (aq)
• Base Acid

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Amphoterism

• Amphoteric capable of acting as either an acid
  or base
• H2O (l) ?

OH- (aq) H(aq)
Al(OH)4- (aq) H(aq)
Al(OH)3 (aq) H2O(l) ?
Acting as an acid
Al(OH)3 (aq) ?
Al(OH)2 (aq) OH-(aq)
Acting as a base

• Amphiprotic can accept or donate a p

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Conjugated Acid-Base Pairs

• For acid HA
• HA (aq) H2O (l) ? A- (aq) H3O (aq)
• acid base

conjugate base
conjugate acid

• For base B
• B (aq) H2O (l) ? HB (aq) OH- (aq)
• base acid

conjugate base
conjugate acid
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Relative Acid-Base Strengths

• The stronger an acid (the greater its ability to
  donate p), the weaker its conjugate base (the
  lesser its ability to accept p).
• The stronger a base, the weaker its conjugate
  acid.
• In an acid-base equilibrium, the p is
  transferred from the strongest acid to the
  strongest base.
• HSO4- CO32- ? SO42- HCO3-

Stronger acid
Stronger base
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16.5 Strong Acids and Bases

• Strong acids and bases fully ionize in water
  (equilibrium is shifted entirely toward ions).
• Strong acids
• HI, HBr, HCl, HClO4, HClO3, H2SO4, HNO3
• Ex In 6M HCl solution, 0.004 exist as
  molecules
• Strong bases
• LiOH, NaOH, KOH, RbOH, CsOH, Ca(OH)2, Sr(OH)2,
  and Ba(OH)2

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16.6 Weak Acids

• Weak acids partially ionize in water (equilibrium
  is somewhere between ions and molecules).
• HA (aq) ? A- (aq) H (aq)

• Ka acid-dissociation constant in water
• Weak acids generally have Ka lt 10-3
• See Appendix D for full listing of Ka values

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• Ex Calculate the pH of 2.0 M HCl solution
  (Ka106)
• Strong acid, completely dissociated
• HCl (aq) ? H (aq) Cl- (aq)

  HCl (aq) H (aq) Cl- (aq)
Initial
Change
Final
2.0 M
0 M
0 M
- 2.0 M
2.0 M
2.0 M
0 M
2.0 M
2.0 M
So HClinitial Hfinal Cl-final 2.0
M pH - log H - log 2.0 -0.30
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• Ex Calculate pH of 2.0 M HF solution
  (Ka7.2x10-4)
• Weak acid, partially dissociated
• HF (aq) ? H (aq) F- (aq)

  HF (aq) H (aq) F- (aq)
Initial
Change
Equilibrium
2.0 M
0 M
0 M
- x M
x M
x M
(2.0 x) M
x M
x M
Using quadratic eqn, 0 x2 7.2 x 10-4x 1.44
x 10-3 x 3.7229 x 10-2 or 3.8669 x 10-2
H pH - log H - log 3.7 x 10-2 1.43
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• Or, since weak acids partially dissociate, assume
  that HFinit gtgt Heq
• Then, HFinit H HFinit

pH - log H - log 3.8 x 10-2 1.42

• General rule if H ? 5 of HA, it is better
  to use quadratic formula.

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• Percent Ionization of an Acid

• Ex Calculate the ionization of
• 2.0 M solution of HCl
• 2.0 M solution of HF

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• Polyprotic acids have more than one H to
  donate
• Ex H2SO3 (aq) ? HSO3- (aq) H (aq)
• Ka1 1st acid-dissociation constant 1.7 x
  10-2
• HSO3- (aq) ? SO32- (aq) H (aq)
• Ka2 2nd acid-dissociation constant 6.4 x
  10-8
• Ka1gtKa2 1st H dissociates more easily than the
  2nd.

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Polyprotic Acids

• Ascorbic acid (Vitamin C)
• Citric acid

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16.7 Weak Bases

• Partially ionize in water.
• B (aq) H2O (l) ? BH (aq) OH- (aq)

Kb base-dissociation constant in water In
practice,
where x OH-
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16.8 Relationship between Ka and Kb

• Weak base NH3(aq) H2O(l) ? NH4(aq)OH-(aq)

Conjugate acid NH4(aq) ? NH3(aq) H(aq)
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• NH3 (aq) H2O (l) ? NH4 (aq) OH- (aq)
• NH4 (aq) ? NH3 (aq) H (aq)

H2O (l) ? H (aq) OH- (aq)
And
Therefore
For a conjugate acid-base pair
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• In general, when two reactions are added to give
  a 3rd, the equilibrium constant for the 3rd
  reaction equals the product of the equilibrium
  constants of the two added reactions.

Furthermore
For a conjugate acid-base pair
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16.9 Salt Solutions as Acids Bases

• Hydrolysis acid/base reaction of ion with water
  to produce H or OH-
• Anion (A-) a conjugate base
• A- (aq) H2O (l) ? HA (aq) OH- (aq)
• Cation (B) a conjugate acid
• B (aq) H2O (l) ? BOH (aq) H (aq)

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Predicting pH of Salt Solutions
Consider the relative strengths of the acid and
base from which the salt is derived
Salt type Cation Anion Hydrolyzes to produce pH

Ca2 conjugate acid of strong base Ca(OH)2
NO3- conjugate base of strong acid HNO3
Strong electrolyte Ex Ca(NO3)2
Neither H nor OH-
7
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Salt type Cation Anion Hydrolyzes to produce pH

Na conjugate acid of strong base, NaOH
ClO- conjugate base of weak acid, HClO
Weak electrolyte Ex NaClO
OH-
gt 7
ClO- (aq) H2O (l) ? HClO (aq) OH- (aq)
where x OH-
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Salt type Cation Anion Hydrolyzes to produce pH

NH4 conjugate acid of weak base, NH3
Cl- conjugate base of strong acid, HCl
Weak electrolyte Ex NH4Cl
H
lt 7
NH4 (aq) H2O (l) ? NH3 (aq) H3O (aq)
where x H
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16.10 Acid-Base Behavior Chemical Structure

• Stronger acids, HA, have
• H with a higher d
• Weaker H-A covalent bond (smaller bond enthalphy)
• More stable conjugate bases A-
• Stronger oxyacids, HxOz-Y, have
• Central nonmetal Y with higher
  electronegativity
• More O atoms
• Ex Rank these in order from strongest to
  weakest HClO, HClO2, HCl, HBr

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16.11 Lewis Acids Bases

• Lewis acid e- pair acceptor
• Brønsted-Lowry acid H donor
• Arrhenius acid produces H
• Lewis base e- pair donor
• B-L base H acceptor
• Arrhenius base produces OH-
• Ex
• NH3 BF3 ? NH3BF3
• Lewis base Lewis acid Lewis salt
• 6 CN- Fe3 ? Fe(CN)63-
• Lewis base Lewis acid Coordination compound

Gilbert N. Lewis(1875 1946)